Hydraulic fluid for a power steering unit is generally delivered by a constant flow rate pump. Fluid flow continues at the prescribed volumetric rate, regardless of system back pressure, so long as the pump is able to deliver the fluid at the prescribed rate. With this constant flow demand there exist risks of pump damage. Therefore pumps for such systems generally are provided with one or more pressure relief mechanisms that terminate the pumping action in case of excessive system loads. The pressure relief mechanisms generally avoid damage to the pump at the expense of temporary impairment of power steering and temporary loss of service from anything else which may be powered by the pump (e.g. driving a cooling fan motor). In many instances, bypass mechanisms are provided around individual components of the system, so as to avoid loss of the entire system when a localized abnormality or overload is experienced.
Proper pump design calls for a hydraulic pump to have only the ability to supply the normal hydraulic fluid needs of the devices which it is serving. If a plurality of devices are being served, it is desirable that they share the hydraulic fluid on some basis of priority. Power steering is clearly a matter of high priority. So long as a power steering unit is operating normally, its needs for hydraulic fluid should receive preferred treatment.
A cooling fan performs an essential function in protecting the automotive engine from overheating. However, the fan operation may be temporarily halted without serious risk to the motor vehicle or to its passengers. Therefore, a cooling fan generally operates in conjunction with a power steering unit on a lower priority basis. In such cases, if the load on the pump becomes excessive, then the needs of the cooling fan may be sacrificed.
Pump performance is generally dependent on engine speed, which is generally expressed in revolutions per minute (RPM). At high engine speed, the hydraulic pump generally produces an output sufficient to provide hydraulic fluid flow for the power steering system and the cooling fan. At lower engine speeds there is generally insufficient hydraulic fluid flow to support both the power steering system and the cooling fan. In many cases, while there is insufficient hydraulic fluid flow at lower engine RPM, there is generally sufficient secondary flow from the priority valve supplying the power steering system to supplement the cooling fan.